The present invention relates to the control of steam turbines, particularly with regard to positioning of the turbine governor valves.
In a steam turbine system, such as utilized in a power generating facility, the power supplied by a turbine is set in accordance with the load demand on the generator, and this setting is effected primarily by controlling the rate of steam flow to the turbine first stage. The rate of steam flow, in turn, is adjusted primarily by appropriate setting of the output pressure from a source of steam, such as a boiler, and appropriate positioning of governor valves via which steam is delivered from the source to the inlet nozzles of the turbine first stage. Such a first stage typically has a plurality of nozzles distributed around its circumference, and a separate governor valve is provided for supplying steam to each nozzle. Depending on the operating requirements of the particular turbine system, all valves can be controlled to operate in unison or in a certain sequence.
Each governor valve can operate between a fully closed state and a fully open state. It is generally desired that each governor valve be placed at one of two selected positions. One of these positions, known as the crack point, is close to the fully closed position, while the other position, commonly known as the knee point, permits nearly full flow through the valve. Operation of a governor valve at a position intermediate the crack point and the knee point is generally undesirable because it results in a pressure drop across the valve, and this has an adverse effect on the efficiency of the turbine and on the heat rate of the power plant.
It is generally considered to be advantageous to operate the turbine governor valves in what is known as the sequential valve mode in which individual valves or groups of valves open or close in sequence as load demand increases or decreases. Particularly when a plant is called upon to operate at less than full load, sequential valve mode operation enhances operating efficiency.
Sequential valve mode operation is characterized by a plurality of governor valve settings which are known as valve points. At each valve point, one or more governor valves are open to a point which permits substantially full steam flow, which is a position between the knee point and a fully open condition, while substantially no steam is flowing through the other governor valves, in that each of these other governor valves is at a position between its crack point and its fully closed state.
If the output pressure from the steam source were maintained constant, each valve point would correspond to a specific load demand level. In order to allow such a system to respond efficiently to load demand levels between those specific levels, it is known to employ the sliding pressuring method in which, for example, the speed of the feed pump supplying water to the steam source, such as a boiler, is reduced. This reduces the pressure throughout the system, starting at the pump outlet, through the boiler, the super heaters and, finally, the turbine stages.
Thus, in one mode of operation of a facility of the type hereunder consideration, response to a given load demand can be achieved by adjusting the boiler output pressure to a value between minimum and maximum permissible values, setting the governor valves to a valve point which is closest to that associated with the selected load demand level and then either increasing or decreasing the boiler output pressure to the value required to meet the selected load demand when the governor valves are set at the selected valve point. Such a technique is described, for example, in U.S. Pat. No. 4,178,762, which issued to Binstock, et al., on Dec. 18, 1979.
In order for a system of this type to operate at optimum efficiency, it is important that all of the governor valves be set to a position closely corresponding to a valve point. While this can be readily achieved in facilities equipped with modern and sophisticated digital controllers which directly monitor the position of each governor valve and create position adjustments on the basis of such monitoring results, many older or less sophisticated facilities are not equipped to monitor the governor valve positions and the addition of monitoring devices to provide valve position feedback signals can add considerably to the cost of modernizing such facilities. However, it is precisely these older and less sophisticated facilities in which the governor valve control system cannot reliably effect precise positioning of the governor valves.